Anisotropic conductive connection can be required to electrically connect connection electrodes of an IC circuit board to terminals of a substrate mounted on a circuit board, such as a liquid crystal display (LCD) panel. Film-type adhesives are widely used as anisotropic conductive packaging materials, in which conductive particles, such as metal-coated resin particles or metal particles, are dispersed in an insulating resin, e.g., epoxy, urethane, or acrylic resin.
Conductive particles are interposed between electrodes and terminals by disposing an anisotropic conductive packaging material containing the conductive particles between the electrodes and the terminals and applying pressure and heat to adhere the packaging material therebetween. Currently, electrical connection occurs in a pressing direction, and an insulation state is maintained in a direction perpendicular to the pressing direction due to the presence of insulating components contained in an insulating adhesive.
In circuit board packaging requiring anisotropic conductive connection, recent advances in circuit technologies have increased connection pitch compactness, IC bump minuteness and the number of leads printed on substrates. Further, there continues to be a need for improved electrical connection reliability.
To satisfy such technical needs, conductive particles contained in anisotropic conductive films are largely required to have a uniform and small particle diameter. Further, it can be critical that conductive particles have enhanced conducting properties without being ruptured, together with good compressive deformability and recoverability from deformation, because of increased contact area with connection substrates when the conductive particles are interposed and compressed between the connection substrates.
Metal particles, such as nickel, gold and silver particles, and metal-coated base particles can be used as the conductive particles. However, since metal particles have a non-uniform shape and a much higher specific gravity than an adhesive resin, they can have poor dispersibility in the adhesive resin.
For these reasons, in mounting applications requiring superior connection of microelectrodes and high connection reliability, conductive particles with a uniform shape and a relatively narrow particle diameter distribution are widely used as a plated layer formed on base polymer particles.
Various proposals have hitherto been made on conductive particles in which polymer particles are plated, and particularly on the characteristics of the particles after compressive deformation in terms of improved contact with electrodes and connection reliability.
For example, PCT Publication WO 92/06402 discloses a spacer for an LCD and conductive particles using monodisperse resin particles as base particles. According to this publication, in order to readily control a gap between electrodes facing each other when the electrodes are connected to each other by compression using the conductive particles, the resin particles preferably have a compression hardness at 10% compressive deformation (10% K value) of 250 to 700 kgf/mm2. In addition, in order to increase the contact area between the conductive particles and the electrodes after compression, the resin particles preferably have a recovery factor after compressive deformation of 30 to 80%.
Further, Japanese Patent Laid-open No. H07-256231 discloses conductive particles having a K value at 10% compressive deformation of 700 to 1,000 kgf/mm2 and a recovery factor after compressive deformation of 65% to 95% at 20° C. in order to improve poor conductivity caused by changes in the temperature between electrodes, folding, mechanical impact, etc.
Moreover, Japanese Patent Laid-open No. H11-125953 and No. 2003-313304 disclose conductive particles having a K value at 10% compressive deformation of 250 kgf/mm2 or lower and a recovery factor after compressive deformation of 30% or greater for better connection reliability.
These patent publications note that as the recovery factor after compressive deformation of the conductive particles increases, the conducting properties of the conductive particles, e.g., increased contact area with the electrodes, are enhanced. In addition, according to the patent publications, the K value reflecting the compressive deformability of the particles is mostly limited to 10% compressive deformation.
The 10% K value of the particles may be a criterion for the initial compressive deformability of the particles. However, when the conductive particles are interposed and compressed between the electrodes, they show a difference of several % to several tens of % in compressive deformation even under the same pressure, depending on the constitution of the base polymer particles, thus leading to different electrical connection and connection reliability due to the difference in the contact area with the electrodes and contact reliability. In other words, although the 10% K values of the particles are within the defined specific ranges, the deformability of the particles may be different under continuous compression. As a consequence, particles having appropriate 10% K value do not necessarily guarantee the provision of conductive particles with increased contact area and superior connection reliability.
Indeed, conductive particles satisfying the 10% K value in a specific range and showing good compressive deformability upon connection to electrodes commonly have a low recovery factor from deformation. Further, such conductive particles are in sufficient contact with electrodes during compression, but show a low recovery factor from deformation after decompression, resulting in poor connection reliability. Moreover, because conductive particles having good compression recoverability are not sufficiently deformed up to several tens of % under common pressures for connection of electrodes, the connection resistance can increase and the connection reliability is likely degraded. To determine whether the connection resistance decreases and connection reliability improves by sufficiently deforming conductive particles by compression, it is necessary to consider K values at several tens of % compressive deformation together with the initial deformability (i.e. 10% K value) and compression recoverability.
Recently, anisotropic conductive adhesive films in which conductive particles are directly dispersed have been fast-cured under selected conditions, e.g., low temperatures and low pressures, within a short period of time in order to connect electrodes to each other. In this case, as the compressive deformability of the conductive particles is decreased, it can be difficult to sufficiently increase the contact area with the electrode surface. As a result, the connection resistance increases and the connection reliability is likely degraded.